Foreign particles on optical surfaces of optical elements (typically mirrors and reticles) for EUV lithography mainly cause uniformity and contrast problems in an EUV lithography apparatus. Besides, there is also a considerable risk that a particle may be transferred to the reticle and/or the waver level where a one to one imaging of the particle may occur. Therefore, there is a need for cleaning foreign particles away from the optical surfaces before integration of the EUV optics in a EUV lithography apparatus or, more preferably, after the integration of the EUV optics inside the EUV lithography apparatus.
United States patent application publication 2009/0231707 discloses an EUV lithography apparatus comprising a device for generating a temperature gradient between the optical surface and the surface of an adsorbent material surrounding the optical surface. The heating of the optical surface, respectively the cooling of the adsorbent surface leads to a reduction of the occupation of contaminating particles on the optical surface, as the temperature gradient induces a movement of the particles from the optical surface to the adsorbent material, also known as thermophoretic effect.
US 2008/0218712 A1 discloses an immersion lithographic apparatus which includes a cleaning system for in-situ cleaning of a component of the lithographic apparatus. The cleaning system is arranged to provide a cleaning environment in proximity of a predetermined position on a component to be cleaned. The system is also arranged to provide the cleaning environment substantially independent of a type of contamination present at the predetermined position.
EP 1 329 773 A2 discloses a lithographic apparatus in which, in a low-pressure environment, a voltage is applied between a tool tip in close proximity to a surface and that surface, and contaminants are attracted and adhere to the tool. A laser may also be used for in-situ cleaning of components of a lithographic projection apparatus.
WO 2004/104707 A2 describes a method for cleaning at least one optical component in an EUV lithography apparatus, the optical component being at least partially soiled with an inorganic substance. At least one reagent which chemically reacts with the soiling deposits is added through a supply system to remove them from the optical component.
US 2007/0132989 A1 discloses an examination system for locating contamination on an optical element. The examination system and the optical element may both be arranged at the interior of the same vacuum chamber, thus allowing in-situ inspection of the optical element without the vacuum in the chamber having to be released.
US 2003/0051739 A1 describes an optical assembly to be mounted in a projection exposure apparatus for EUV microlithography having a mirror and a cleaning a device. US 2004/0011381 A1 discloses a method for removing carbon contamination from optical surfaces using atomic hydrogen.